This invention generally relates to devices for testing the effectiveness of an insulating interface within an electrodynamic machine, and is specifically concerned with both a system and a method for detecting faults in the insulation disposed between the shaft and the bearings and seals of an electric generator. In electric generators, the generator and exciter bearing pedestals and generator seals must be insulated to prevent unwanted voltages induced in the generator shaft from creating electric arcs across the generator seals and bearing oil films. Such voltages are created both by electrostatic generation,, and by dissymmetry currents created by non-uniformities in the magnetic fields in the stator. Of the two types of currents, the dissymmetry currents are potentially more destructive, as the amperage associated with such currents is high enough to destroy the generator seals and bearings in a matter of hours. Under normal operating circumstances, the ASGS (active shaft grounding system) helps to prevent the creation of such arcing by means of three conductive brushes which maintain electrical contact with the shaft as it rotates. When these brushes detect that a current of between about 40 and 80 milliamps has begun to flow through the bearing pedestal and shaft seal insulation, they ground out the generator shaft, and at the same time activate an alarm which informs the generator operation that an insulation failure has occurred. The generator unit is then removed from service and disassembled to be fixed.
Unfortunately, the ASGS is only capable of detecting insulation failures when the generator is in service. Moreover, it can only tell the generator maintenance crew of the existence of such a failure, and not the precise location where the failure occurred. Hence, if the generator is taken off-line for insulation repairs, there is no sure way to tell whether or not the repairs were completely effective in reinstating the insulation until the generator is actually placed into active service again. To increase the probability of a successful repair, it has been proposed to merely install double layers of insulation throughout all of the suspect areas. However, such a solution is expensive not only in terms of the effort needed to install a second complete layer of insulation, but also due to the fact that double insulation requires the installation of additional insulation oil line connections to the bearings which increase the chances of an oil leak failure.
Clearly, there is a need for a system which accurately and reliably detects failures in the insulating interface between the generator shaft and the bearing pedestals and seals which rotatably support it that obviates the need for applying double layers of insulation. Ideally, such a system would be able to locate the precise location of such a failure to obviate the need for replacing all such insulation.